The oxidative theory of aging proposes that declines in cellular function associated with aging are due to the accumulation of damaged macromolecules. Mitochondria are the main source of endogenous reactive oxygen species (ROS), which inflict damage to macromolecules, particularly to mitochondria! DNA (mtDNA). DMA lesions are deleterious to cells because they halt transcription, replication, and can generate mutations. Cells have evolved several mechanisms to deal with oxidative damage. One line of defense is represented by the antioxidant defense system, which includes among others, the superoxide dismutase (SOD) enzymes. The second line of defense is DNA repair, particularly base excision repair (BER). Although the enzymatic components of both defense systems are known, it is not understood how the systems interact to maintain mitochondrial genomic stability. The main hypothesis in this research proposal is that the BER pathway and the antioxidant defense system have cooperative roles in protecting against mitochondrial genomic instability induced by oxidative stress. Oxidative stress will be considered under two situations: during aging and after an environmentally induced insult. To test this hypothesis we will use two model organisms, a simple eukaryotic organism, the yeast Saccharomyces cerevisiae, and a more complex mammalian organism, a knockout mouse heterozygous for Ape1, the main AP endonuclease that participates in BER. The following specific aims will be used as criteria to test our hypothesis: (1) To determine whether yeast cells harboring simultaneous mutations in the APN1 and SOD2 genes exhibit enhanced mtDNA damage and mitochondrial dysfunction after oxidative stress;(2) To determine whether the levels of mtDNA lesions increase in an age dependent fashion in yeast cells harboring simultaneous mutations in the APN1 and SOD2 genes, and if levels of mtDNA lesions correlate with mitochondrial dysfunction;(3) To determine whether mitochondrial DNA damage increases in an age-dependent fashion in a BER deficient mouse model. Finally, we propose a developmental plan whose objective is to lead the PI to improve his publication record and generate the preliminary data necessary for the submission of non-SCORE competitive research grants.